1. Technical Field
The present invention relates generally to analytical spectroscopy and, more particularly, to apparatus and methods for improving the detection and measurement of spectra.
2. Background Art
The use of multichannel detectors in ultraviolet, visible and near-infrared spectroscopy has become widespread because of the inherent advantages multichannel detectors have over single channel detectors. The well known multichannel advantage arises because information at multiple wavelengths can be recorded simultaneously. The advantage is proportional to the square root of the number of simultaneous measurements. The charge-coupled device (CCD) has become particularly popular as a spectroscopic detector because scientific grade versions of these devices, when operated in slow-scan cameras, provide individual array element sensitivities that rival the best single channel detectors.
Large format two-dimensional CCD arrays have a sufficiently large number of pixels (picture elements) to meet the resolution requirements of even the most demanding analytical spectroscopic technique assuming that the spectrum can be properly composed to make effective use of the CCD. Unfortunately, the physical size of the individual device elements (at most a few tens of microns) makes it difficult to design a spectrometer which makes efficient use of the entire CCD array. For this reason, most spectroscopists currently use CCD array detectors (multichannel detectors) with spectrometers designed for single channel detectors such as photomultiplier tubes. These spectrometers disperse light in one direction, so one axis of a two-dimensional CCD array is used only to collect light from along the height of entrance slit images at the focal plane High sensitivities are achievable with this arrangement, without the use of cylindrical lenses or other optical modification, if the effective pixel size is matched to the height of the slit images. Matching is accomplished by combining the photogenerated charge from adjacent pixels prior to readout (charge binning).
This arrangement has a substantial drawback in that a compromise between resolution and wavelength coverage is often required when using readily available CCD arrays having sizes of 512, 576, or 1024 pixels on a side. Depending on the particular spectroscopic technique, these sizes may not be sufficient to cover the desired wavelength range at adequate spectral resolution. For example, it is desirable in atomic emission spectroscopy to have resolution of 0.01 nanometers over the ultraviolet and visible spectral regions (180 to 800 nm). A 620 nm wavelength range covered at 0.01 nm resolution requires 124,000 resolution elements to satisfy the Nyquist Criterion.
The traditional approach to dealing with the resolution/wavelength coverage dilemma is to observe smaller spectral regions and to piece the resulting spectra together. Aside from the difficulties associated with maintaining wavelength calibration, problems associated with throughput variations (as a function of diffraction angle) produce undesirable baseline effects, resulting in discontinuities at the individual spectral boundaries. A recent report by P. Knoll, R. Singer, and W. Kiefer, entitled Improving Spectroscopic Techniques by a Scanning Multichannel Method, Appl. Spectros. 44, 776-782 (1990), proposes a partial solution to this latter problem in a photodiode array based instrument. The authors demonstrate collecting spectra at much smaller wavelength intervals than those mandated by the size of the array. The spectra are added in regions of overlap so that the variations in throughput tend to average out. The authors reduce spectral collection time in proportion to the increase in the number of spectra collected so that the overall measurement time is held constant.
The unique readout capabilities of CCDs arrays allows for a much more elegant approach to compensating for throughput variations than is possible in photodiode arrays. The general readout process in a CCD array involves shifting the photogenerated charge row by row to a serial register which then allows the sequential readout of pixels within each row. One mode of operating a CCD enables acquisition of an image which exceeds the physical size of the CCD array (in one dimension) by an arbitrary amount. This mode is called Time-Delay Integration (TDI). In this operating mode, the CCD array is exposed to a continuously moving image and the shifting of charge is synchronized with the movement of the image. The only restriction is that the direction of image movement be parallel to the column axis of the array and towards the serial register. TDI operated CCD arrays were first used in airborne reconnaissance in the early 1970's.
To the Applicant's knowledge, operating a CCD array in the TDI mode for the collection of spectra in analytical spectroscopy has not been done. TDI operation of CCD cameras is widely known in the remote image sensing field. More recently, TDI operated CCDs have been applied to the inspection of moving webs, as reported in an article by D. L. Gilblom, TDI Imaging in Industrial Inspection, Sierra Scientific, 1173 Borregas Ave, Sunnyvale, Calif. 94089, and in U.S. Pat. No. 4,922,337 to Hunt et al. In addition, a TDI operated CCD array has been used in analytical chemistry as described by J. V. Sweedler, J. B. Shear, H. A. Fishman, R. N. Zaire, and R. H. Scheller, Fluorescence Detection in Capillary Zone Electrophoresis Using a Charge-Coupled Device with Time-Delayed Integration, Anal. Chem. 63, pp. 496-502. In this application, the TDI operated CCD array is used to follow moving components separated from a mixture by capillary zone electrophoresis. A spectrograph is inserted between the image forming lens and the array so that the entire spectrum of the components can be acquired as each row of the CCD array is read. The spectrum is not detected by time-delay integration of the constituents of the spectrum.
The present invention applies the TDI mode of operation to analytical spectroscopy and eliminates the restrictions placed on wavelength coverage and resolution by finite CCD array sizes. In addition, the present invention avoids the problems associated with variations in throughput as a function of diffraction angle. Moreover, the present invention obtains the advantages of multichannel detection while benefiting from the simplicity and flexibility of available single channel spectrometers.